Right now, gardeners in much of the southern and western portions of the country are dreading the month of November, which traditionally brings the first killing frost. For gardeners in the country's coldest climates, the first plant-blackening frost may already have put an end to the season.
Fall is an interesting time for gardeners. Temperatures may dip into the 30s and lower, only to rise dramatically during the day. The sad part is, once frost has killed growing plants, for all intents and purposes the season is over.
Although this relentless march of weather is inevitable, there are ways to eke a bit more out of a garden. Cool-weather crops such as broccoli, cabbage, kohlrabi, and pansies will continue to grow until temperatures are consistently well below freezing. Plants that are located in protected spots such as cold frames can continue growing, albeit more slowly, even when the first snows carpet the ground. And for gardeners who are trying to cheat Mother Nature on a grander scale, perennial plants that aren't considered hardy in an area might survive the winter if you take special steps to protect them through the coming months.
Gardeners have long practiced both low-tech and increasingly high-tech ways to cheat Jack Frost. The simplest short-term way to do this on cold fall nights is to lay old bed sheets or blankets over prized plants. The shelter will help trap the heat and moisture that radiates from the earth, providing a few scant degrees of difference between a cool night and a cold one.
On a slightly more high-tech level, gardeners can use rowcovers to protect their plants. Rowcovers are woven fabrics that can be stretched over a form to provide tunnels that will protect a whole row of plants. Because they're made from fabric, rowcovers allow air and water, as well as light, to reach the plants. (Rowcovers are also used as early-spring pest controls because the fabric forms a barrier that the insects can't penetrate.) A commerical rowcover, such as those made by companies like Reemay or Agryl, should protect plants against temperatures as low as 28 degrees F.
Gardeners are also ingenious when it comes to devising protection for plants much too large for either rowcovers or sheets. One gardener in California protects a well-loved orange tree during particularly chilly nights by turning on a simple string of Christmas lights that has been woven through the branches. The slight heat emitted by the bulbs is enough to raise the temperature around the tree by a few degrees.
Commercial orange growers attempt the same technique, on a much grander scale. They set up portable heaters among orchard rows to raise the air temperature slightly on particularly cold nights, or employ sprinkler systems to mist the trees throughout the night to provide a bit more warmth. (Which brings up an interesting point: Some gardeners swear that if a light, unexpected frost touches unprotected plants, the plants can still be saved if they are watered with a sprinkler early in the morning before the sun touches the leaves. Does this work? In an emergency, it's worth a try.)
Cold frames, hot beds
If you live in a warmer area of the country, squeaking plants through one or two nights of cold weather is one thing. For gardeners who live where the temperatures get and stay cold, adding to the season becomes a whole different matter. That's where more permanent season extenders come into play.
One of the most popular of these options is the cold frame, both to get a jump on the season in spring, and to help extend the season in fall. Cold frames can also be used to overwinter less-hardy perennial plants by providing additional protection from wind and cold. Traditionally, cold frames are best used for growing cool-weather plants such as pansies or cool-season vegetables.
A cold frame is an outdoor structure that works much like a miniature greenhouse. At their simplest, cold frames are hinged, partially buried "boxes" dug slightly into the ground. The cold frame has a lid made of a transparent material such as glass or plastic to admit light. Cold frames can be built from a number of different materials--concrete, wood, brick, or even carefully arranged straw bales. Most cold frames face south, to admit as much light as possible. Insulation from both the frame's construction and the surrounding earth can provide protection to enclosed plants for far longer than those same plants would survive if planted in an unprotected garden.
On hot, sunny days, the lid can be raised slightly to prevent plants from "cooking" in the heat.
A variation on the cold frame is something known as the hotbed. Similar to a coldframe in structure, hotbeds provide even more protection because they have an internal source of heat.
Today gardeners who employ hotbeds may use electric cables to heat the growing area. However, in years past, when fresh manure was a product easily available to home gardeners, a low-tech way of building a bed with a self-generating "heating system" called for a frame and lid to be constructed over a thick pile of uncomposted manure. As the manure rotted, it produced heat. Problem was, the heat that emanates from such a bed is often irregular, ranging from very hot early in the process, to almost negligible at other times. If, however, you'd like to try experimenting, here's how to build a composting hotbed:
Select a sheltered spot that has a southern exposure. Dig a hole about 30 inches deep in a shape that will accept your hotbed cover. Add a few inches of gravel to the bottom of the hole to improve drainage. Now add a layer of uncomposted manure about 18 inches deep, over the top of the gravel. Fill in the hole with good well-drained soil to ground level. A good thermometer should be placed in the soil to help you gauge the composting process.
According to Denny Schrock, a horticulturist with the University of Missouri-Columbia, when the process first begins, the soil above the manure may reach temperatures as high as 100 degrees F; it's best to wait until the soil temperature has cooled to at least 70 degrees F to add plants or seedlings. Remember that hotbeds must have excellent drainage. If the manure stays too wet, it won't begin to decompose.
"These strategies are all well and good for those people who live in warmer areas," huff gardeners in areas where temperatures plunge well below zero for weeks at a time, "but what about me?"
To cheat Mother Nature in truly cold climates, nothing works but to prepare plants to live outside through the winter, to bring them inside, or to use a greenhouse.
To keep perennials going strong through the winter, the most important factor is to ensure they are well watered before going into this stressful time. This is especially important for evergreens, which can lose a great deal of moisture to drying winds through their leaves. You should also stop fertilizing your perennials in midsummer since fertilizer pushes plants to grow; in autumn they need to stop growing and start hardening off. If plants aren't hardened off properly, they're much more susceptible to winter injury.
The other cold-weather survival standard is to apply a thick layer of mulch to beds after the ground has frozen.
Mulching works, but not necessarily the way you think. The real damage to most herbaceous plants (non-woody ones like hostas that die all the way to the ground), comes from heaving. Heaving occurs when the soil freezes and then thaws in a cycle. Moisture in the soil freezes and then expands and pushes the soil up and out, away from plant roots. Mulch interrupts this cycle by maintaining a steady soil temperature. Once your soil has frozen, you want it to stay that way until spring. (A general guide is to wait to put your mulch on until the daytime temperatures don't rise above 32 degrees F.)
The best mulch to use is largely a matter of choice, although you shouldn't use a material such as unshredded leaves, because they tend to form solid mats. Since moisture can't evaporate from under these mats, both disease and rot can become problems. Gardeners in the coldest parts of the country can add as much as 6 to 12 inches of mulch; those in milder areas might only need an inch or two. Snow is also an excellent mulch in heavy snow areas.
Once you've protected the plants that will stay outside for the winter, it's time to bring in those favorites that won't survive, even with mulching.
Gardeners have been growing pampered plants in pots for years and then bringing them into either a heated greenhouse or indoors. One common container - grown plant that is often overwintered indoors is a nonhardy fruit tree. At one point in time, the ultimate in sophistication was to have an "orangerie" or orange house attached to European estates. In this room, dozens or even hundreds of orange trees were carefully pampered through the winter, often bearing fruit indoors during cold months.
One important key to growing plants such as fruit trees in containers is to be sure you have the right size pot for your plant. Lon Rombough, an avid specialty fruit grower in Oregon, says that for many dwarf fruit plants, a 5-gallon pot does nicely but for larger plants such as dwarf apples, you might need a pot as large as 10 gallons.
The ultimate "zone-beating" strategy, of course, is a greenhouse. With the right set of conditions, gardeners in any zone can grow tropicals such as hibiscus, bougainvillea, and passionflowers all year.
Jeanne Gustafson, greenhouse manager at Charley's Greenhouse Supply Company in Mount Vernon, Washington, says there are two basic categories of greenhouse: those that are attached to a house and those that are freestanding. One advantage to an attached structure is that it retains heat a little bit better than a freestanding greenhouse. In addition, the type of "glazing," or building material used in a greenhouse, will affect its ability to overwinter plants.
Some materials, such as single layers of glass, provide very little protection in comparison to materials such as "twinwall," a clear plastic that has air channels between the top and bottom of its walls. As a general rule, Gustafson says, an unheated greenhouse made from twinwall will stay about 5 to 7 degrees warmer than the outside temperature. A glass-walled greenhouse will be about the same temperature as the outside air unless it receives supplemental heating.
Whatever your season-beating strategy, the last few days in the garden can be as precious as the first few days in spring.